Effect of Processing Variables on the Structural Evolution of Silica Gels

Constant shear rate and dynamic rheological measurements have been used to investigate the effect of shear on the processes controlling the structural evolution (size and volume fraction of clusters and the extent of cluster–cluster cross-linking) during the gelation of colloidal silica sols. In the absence of shear, the storage and loss moduli (G′ and G″, respectively) initially increase slowly prior to gelation, indicating that cluster growth and network formation are initially proceeding slowly, but then the system evolves rapidly, with cluster growth occurring at a slightly faster rate than network formation. In contrast, sols presheared for 4 h prior to gelation exhibit rapid increases in both G′ and G″ immediately after cessation of the applied shear, reflecting significant differences in the evolution of the gel structure. On aging, the viscoelastic properties of the unsheared and presheared samples are similar, indicating that their structures are comparable on the length scales (several microns) being probed by the frequency range used here. However, their chemical and microstructural properties differ significantly, due to differences in the intercluster bonds. The effect of shear rate, initial pH, colloid particle size, and volume fraction on the structure of the resulting sols and gels is discussed.